1. Field of Invention
This invention relates generally to vortex type flowmeters for measuring the rate of flow of a fluid passing therethrough, and more particularly to a hybrid sensing system for a meter of this type which renders the meter operative throughout a very wide range of flow rate values.
2. Prior Art
A vortex-type flowmeter is adapted to measure the flow rate of a fluid passing through a conduit by producing fluidic pulses or oscillations whose repetition rate or frequency varies in accordance with flow rate. Two species of vortex-type meters are commercially available in the United States, one being the so-called Swirlmeter type, and another, the bluff-body type. The present invention is not limited to these specific types and is applicable to all forms of hydrodynamic oscillatory metering devices in which fluidic variations are sensed to provide an indication.
In Swirlmeters, such as those described in U.S. Pat. Nos. 3,279,251, 3,314,289 and Re. No. 26,410, among others, a fluid whose flow rate is to be measured, is forced to assume a swirl component. This is accomplished by feeding the fluid into the inlet section of a flow tube having a fixed set of swirl blades therein which imparts a swirling motion to the fluid passing therethrough. Downstream of the swirl blades in the tube is a Venturi section to create a vortex.
Precession takes place about the central axis of the flow tube at a discrete frequency that is a function of the volumetric flow rate. Cyclic variations in local fluid velocity occurring by reason of precession are sensed to provide electrical pulses whose frequency is measured to provide an indication of flow rate.
In bluff-body type vortex meters as disclosed, for example, in my prior U.S. Pat. Nos. 4,123,940; 4,181,020; 4,226,117; 4,262,544; 4,433,957 and 4,270,391, whose disclosures are incorporated herein by reference, a shedder body is mounted in the flow tube which acts to divide the incoming fluid into two streams and to cause vortices to be alternately shed on either side of the body.
In making flow rate measurements, very large variations in flow rate are encountered in certain commercial and industrial applications. One such application involves flow rate measurement of natural gas. In a typical installation, the natural gas to be metered is used to supply a gas-fueled furnace as well as gasfired stoves and other appliances.
In wintertime, the furnace, whose operation is regulated and therefore undergoes an on-off cycle, usually represents the main load in the gas supply line and draws more than 60 percent of the gas being metered. The remaining base load might vary from 2 to 10 percent at various times. In order to measure this widely varying flow properly, a flowmeter would be required with a better than 50 to 1 operating range.
Vortex meters, whether of the Swirlmeter or bluff-body type, are potentially fully capable of operating within a very wide range of flow rate values, but what prevents them from doing so are the existing limitations of their sensors to detect fluidic pulses or oscillations.
As will be evident from the patents identified above which disclose Swirlmeters as well as bluff-body types of vortex meters, a number of different types of sensors have heretofore been provided to detect the passage of fluidic vortices or the presence of fluidic oscillations in the flow tube. In the case of Swirlmeters, the most commonly used sensors are of the thermal type, while in the case of bluff-body vortex meters, use is often made of force type sensors.
Thermal sensors are constituted by an element whose resistance varies as a function of temperature. In practice, the thermal sensor may be a self-heating element, such as a platinum or nickel wire or film, a silicon element, or a thermistor. Or it may take the form of an indirectly-heated device such as a thermocouple.
The thermal sensor is exposed to the fluidic oscillations in the meter, and as the velocity of the fluid passing the body of the electrically-heated sensor increases, this action serves to withdraw heat from the sensor. But when the fluid velocity decreases, less heat is removed and the sensor heats up. These thermal changes are reflected in corresponding changes in resistance which are converted in an associated circuit into a signal indicative of the frequency of the oscillations and hence of the prevailing flow rate.
Since heat transfer is a relatively slow process, thermal sensors are incapable of changing temperature instantaneously and require a finite time to effect this change. In general, the larger the mass of the sensor, the slower is its thermal response and the smaller the change in body temperature in a given time period.
At high fluidic velocities (i.e., high frequencies--for many vortices then pass by), one must use a thermal sensor of very small mass in order to realize measurable temperature fluctuations to produce an output signal that is proportional to the fluidic frequency. The difficulty with thermal sensors having a tiny mass is that they tend to be fragile and are therefore not well suited to industrial applications. On the other hand, thermal sensors of larger mass, though inherently more rugged, are effectively responsive only to the low velocity range.
A force sensor which may take the form of a wire strain gauge, a piezoelectric pressure-responsive assembly or a pressure-responsive electromagnetic transducer, exploits the pressure and velocity phenomenon associated with fluidic vortices to detect their passage past the sensor. The forces generated by the vortices vary as a function of density multiplied by the square of the velocity. Consequently, at low velocities, these forces are very small, and a force sensor adapted to detect these small forces tends to be too fragile to reliably sustain the forces generated by dense fluids at high velocity. On the other hand, it is relatively easy to design a rugged force sensor to detect high-velocity, high-density fluids.
Thus a rugged thermal sensor having a relatively large mass suitable for commercial and industrial applications, responds best to fluidic oscillations or vortices in the low-frequency operating range, while a rugged force sensor suitable for the same application responds best to fluidic oscillations in the high-frequency operating range. Neither sensor, by itself, is capable of responding to fluidic oscillations in a wide operating range that encompasses both the low-frequency and high-frequency ranges.